1. Field of the Invention
The present invention relates to a method for preparing a single crystal of lanthanum cuprate.
2. Description of the Prior Art
Lanthanum cuprate (La.sub.2 CuO.sub.4) is an oxide having a potassium nickel fluoride (K.sub.2 NiF.sub.4) type structure and exhibits superconductivity at a low temperature when a part of La atoms is substituted by Ba or Sr atoms. The transition temperature thereof is within the range from 30.degree. to 40.degree. K. and thus a variety of applications to devices operating at a very low temperature will be conceivable for the future. For this reason, it has been required to prepare single crystals having a large scale and exhibiting a good quality as well as to develop a method for preparing such a single crystal.
A photograph of such a crystal having a size of the order of 5.times.5.times.1 mm grown by a conventional method has been published (Y. Hidaka et al.; J. Appl. Phys., 26, L377 (1987)), however, the details of the process has not been published.
Heretofore, the flux method has been employed to obtain single crystals of a material which decomposes at high temperature and cause peritectic reaction during cooling and thus cannot provide single crystals thereof by a usual pulling method (Czochralski method). The flux method in general comprises the steps of mixing an objective material with a flux, melting the mixture thus obtained, slowly cooling the molten mixture to bring it into a supersaturated state and thus to crystallize single crystals of an objective material from the melt. In this connection, the kind of flux for crystallizing the objective single crystal and the amount thereof to be added to the objective material as well as the temperature range for the growth of the single crystal must be made clear in order to grow with certainty single crystals by the flux method. For that purpose, it is needed to obtain a phase diagram of a system composed of a raw material and a flux.